1. Field of the Invention
The present invention relates to a photovoltaic generator with an array of multiple parallel-connected strings of series-connected photovoltaic modules, wherein a portion of the photovoltaic modules of a string can be short-circuited by means of a shorting switch, the activation of which takes place when a predefined voltage value across the string is exceeded.
2. Description of the Background Art
Photovoltaic systems of this nature are extremely well known. As a general rule, these systems are constructed such that a plurality of strings are connected in parallel. In these designs, the maximum number of strings is based on the output of the inverter to which the strings are connected. Modern inverters can be designed for a DC input voltage of up to approx. 900 volts.
At the present time, it is customary to construct each string in the system from eight photovoltaic modules, each of which has 60 photovoltaic cells. Thus a total of 480 cells are connected in series with one another. In the open-circuit case, a voltage of 1.5 volts is present at each cell, resulting in a string voltage of 720 volts, which is considerably below the maximum voltage of 1000 volts specified by the manufacturers of the modules. If a higher voltage is present, this can lead to destruction of the modules and the entire system.
During operation of the system, the open-circuit voltage of the cells drops to an operating voltage of approximately 1 to 1.1 volts, so that a voltage between 480 volts and 510 volts is present between the ends of the conventional strings. In the example shown in the figures that follow, an operating voltage of 1 volt per cell is assumed for the sake of simplicity, hence a voltage of 60 volts across a single voltaic module with 60 cells. In the event that the operator of the grid to which the photovoltaic system is connected should disconnect it from the grid for any reason (e.g. a short circuit in the supply cable), the voltage jumps to the aforementioned 720 volts, which is not critical for the modules or the system.
On the other hand, it would be desirable to operate the photovoltaic modules and also the inverter with a voltage higher than 480-510 volts in normal operation, ideally at the maximum permissible voltage of 1000 volts. This is not possible, however, since a voltage of approximately 1500 volts in the open-circuit case would lead to the destruction of the photovoltaic modules, the inverter, and the system.
For operating the photovoltaic system at a higher operating voltage, it is known from DE 3041078 to employ a shorting switch that short-circuits a portion of the modules in the event that an overvoltage arises.
For a large system with hundreds of arrays, however, this measure entails high expenditures for wiring and switches. Some of the arrays are located hundreds of meters apart from one another, and there is an additional need for several kilometers of cable that must be laid and connected.